There are an increasing number of applications that require current sensing and current regulation with inductive loads. These applications include, but are not limited to: switching motor drivers, voltage regulators, and rectifying circuits and systems.
Switching applications such as these often employ an H-bridge or half-bridge to drive the inductive load. In certain applications, the system attempts to halt or reverse the direction of current though the inductive load by switching transistors in the bridge on and off. This can cause a so-called fly-back effect to occur, where current through the inductive load cannot change direction instantaneously, causing a phase discrepancy between the switching bridge (which may effectively provide alternating power to the load) and the current through the load. Immediately after switching, the fly-back effect can cause a reverse current to flow from ground, through the low side transistors, to the load.
Phase discrepancies and the fly-back effect can lead to imprecision in controlling the load. For instance, in the case of a motor controller, phase discrepancies can cause imprecision in controlling the motor. In the case of a voltage regulator or rectifier, phase discrepancies can create unwanted voltage or current levels in the output. Thus, some systems attempt to regulate fly-back voltage and current to minimize their effect. To do so, some systems monitor and detect fly-back current that flows in a reverse direction through the low side transistors of the H-bridge or half-bridge.